With increased environmental restrictions placed on valves, valve stem packing has been identified as a major source of pollution in process plants and the like.
While manually controlled valves pose a lesser problem because (a). the packing can be compressed tighter without concern for excessive stem friction and (b). they are operated less frequently, automatic modulating control valves have more servere problems. An over-tightening of a reciprocating type stem packing will cause immediate and excessive operator hysteresis. The wear of such a packing is also more pronounced because modulating or throttling control valves may operate continuously.
One common solution to this problem has been to employ metallic bellows seals around the reciprocating valve stems. This solution is very costly and in addition, such bellows are subject to pressure limitations and fatigue life restraints.
Pre-adjusted or pressure assisted packing, consisting typically of spring loaded chevron or v-rings made from TFE resins, offer better seals with less friction than conventional woven, solid rings made from asbestos, TFE-yarn or graphite. However, they are subject to wear, particularly when the valve stem is subject to internal or external corrosion or adherence by solid particles.
My invention overcomes these and other problems by providing a combination of packings in one single assembly. While the lower portion of my dual packing system retains the more rugged and adjustable conventional woven solid packing rings which are less subject to wear and which may be re-tightened in service, the upper portion of my invention will consist of pressure assisted v-rings made from solid TFE, for example. The packing follower separating the two sets is slidingly arranged to enable compression of the lower rings without compressing the upper rings. An additional o-ring seal will prevent leakage to pass the exterior of the packing follower.
A user is now able to compress the lower packing at will without putting excessive pressure on the upper v-ring packing (which would cause blockage of the valve stem). It also permits to add a skive-cut woven packing ring for emergency repairs without disassembling the valve, by simply lifting the follower out of the packing box area. While the lower woven packing will wipe and clean the valve stem from any foreign matter, the upper v-ring packing will serve as a back-up and prevent leakage of any fluid that may pass the first or primary packing.